The present invention is directed to code-division multiple-access cellular-communication systems. It particularly concerns the arrangement of cells used to implement such systems.
It has been recognized that a code-division multiple-access approach to providing cellular communications affords significant advantages over competing approaches. The advantages largely concern system capacity. In conventional approaches, the system resources, such as frequency or time slots or both, are disjoint and tend not to be used efficiently. For instance, although a given conventional cellular system may employ a relatively large number of frequencies to provide different channels, the need to avoid interference between adjacent cells means that the channel frequencies employed in one cell cannot as a practical matter be employed in an adjacent cell. So systems that can employ only one-seventh of the allocated bandwidth in any given cell are not uncommon. Also, conversational dead passages usually command just as much of such systems' capacities as active passages do; to take advantage of such dead passages would require fast switching, detection of the dead spaces, and rapid allocation to other users, all of which are difficult from a practical standpoint.
Code-division multiple-access (CDMA) systems suffer much less from these constraints. The typical code-division multiple-access (CDMA) system employs a common frequency band for all channels, the distinction among channels being the code. A CDMA transmitter uses a pseudorandom spreading code to spread a given channel's energy throughout the CDMA frequency band so that the result appears to be bandlimited white noise. A CDMA receiver applies a reverse, despreading code to the wide common frequency band. This compresses the channel of interest's (broadband) contribution into a narrow band in which the signal can be detected. Other channels are similarly transmitted but with different codes, so a given channel's receiver just respreads the contributions from other channels, which remain wideband noise.
This noise imposes the only limit to the number of CDMA channels in a given frequency band; the number of possible codes is typically large enough not to limit system capacity. Since the major limiting factor is the total power received from the other channels, the system can readily take advantage of conversational dead time by employing a modulation scheme in which dead time requires little or no transmission power. More important, the large number of possible spreading codes means that all channels in adjacent cells interfere only as noise, so all cells can use the entire allocated spectrum. Because of these advantages, a CDMA system theoretically provides many time as much capacity as non-CDMA systems of the same bandwidth.